Field of the Invention
The present invention relates to bioresorbable scaffolds; more particularly, this invention relates to bioresorbable scaffolds for treating an anatomical lumen of the body.
Description of the State of the Art
Radially expandable endoprostheses are artificial devices adapted to be implanted in an anatomical lumen. An “anatomical lumen” refers to a cavity, or duct, of a tubular organ such as a blood vessel, urinary tract, and bile duct. Stents are examples of endoprostheses that are generally cylindrical in shape and function to hold open and sometimes expand a segment of an anatomical lumen. Stents are often used in the treatment of atherosclerotic stenosis in blood vessels. “Stenosis” refers to a narrowing or constriction of the diameter of a bodily passage or orifice. In such treatments, stents reinforce the walls of the blood vessel and prevent restenosis following angioplasty in the vascular system. “Restenosis” refers to the reoccurrence of stenosis in a blood vessel or heart valve after it has been treated (as by balloon angioplasty, stenting, or valvuloplasty) with apparent success.
The treatment of a diseased site or lesion with a stent involves both delivery and deployment of the stent. “Delivery” refers to introducing and transporting the stent through an anatomical lumen to a desired treatment site, such as a lesion. “Deployment” corresponds to expansion of the stent within the lumen at the treatment region. Delivery and deployment of a stent are accomplished by positioning the stent about one end of a catheter, inserting the end of the catheter through the skin into the anatomical lumen, advancing the catheter in the anatomical lumen to a desired treatment location, expanding the stent at the treatment location, and removing the catheter from the lumen.
Scaffolds and stents traditionally fall into two general categories—balloon expanded and self-expanding. The later type expands (at least partially) to a deployed or expanded state within a vessel when a radial restraint is removed, while the former relies on an externally-applied force to configure it from a crimped or stowed state to the deployed or expanded state.
Self-expanding stents are designed to expand significantly when a radial restraint is removed such that a balloon is often not needed to deploy the stent. Self-expanding stents do not undergo, or undergo relatively no plastic or inelastic deformation when stowed in a sheath or expanded within a lumen (with or without an assisting balloon). Balloon expanded stents or scaffolds, by contrast, undergo a significant plastic or inelastic deformation when both crimped and later deployed by a balloon.
In the case of a balloon expandable stent, the stent is mounted about a balloon portion of a balloon catheter. The stent is compressed or crimped onto the balloon. Crimping may be achieved by use of an iris-type or other form of crimper, such as the crimping machine disclosed and illustrated in US 2012/0042501. A significant amount of plastic or inelastic deformation occurs both when the balloon expandable stent or scaffold is crimped and later deployed by a balloon. At the treatment site within the lumen, the stent is expanded by inflating the balloon.
The stent must be able to satisfy a number of basic, functional requirements. The stent (or scaffold) must be capable of sustaining radial compressive forces as it supports walls of a vessel. Therefore, a stent must possess adequate radial strength. After deployment, the stent must adequately maintain its size and shape throughout its service life despite the various forces that may come to bear on it. In particular, the stent must adequately maintain a vessel at a prescribed diameter for a desired treatment time despite these forces. The treatment time may correspond to the time required for the vessel walls to remodel, after which the stent is no longer needed.
Examples of bioresorbable polymer scaffolds include those described in U.S. Pat. No. 8,002,817 to Limon, U.S. Pat. No. 8,303,644 to Lord, and U.S. Pat. No. 8,388,673 to Yang. FIG. 1 shows a distal region of a bioresorbable polymer scaffold designed for delivery through anatomical lumen using a catheter and plastically expanded using a balloon. The scaffold has a cylindrical shape having a central axis 2 and includes a pattern of interconnecting structural elements, which will be called bar arms or struts 4. Axis 2 extends through the center of the cylindrical shape formed by the struts 4. The stresses involved during compression and deployment are generally distributed throughout the struts 4 but are focused at the bending elements, crowns or strut junctions. Struts 4 include a series of ring struts 6 that are connected to each other at crowns 8. Ring struts 6 and crowns 8 form sinusoidal rings 5. Rings 5 are arranged longitudinally and centered on an axis 2. Struts 4 also include link struts 9 that connect rings 5 to each other. Rings 5 and link struts 9 collectively form a tubular scaffold 10 having axis 2 represent a bore or longitudinal axis of the scaffold 10. Ring 5d is located at a distal end of the scaffold. Crown 8 form smaller angles when the scaffold 10 is crimped to a balloon and larger angles when plastically expanded by the balloon. After deployment, the scaffold is subjected to static and cyclic compressive loads from surrounding tissue. Rings 5 are configured to maintain the scaffold's radially expanded state after deployment.
Scaffolds may be made from a biodegradable, bioabsorbable, bioresorbable, or bioerodable polymer. The terms biodegradable, bioabsorbable, bioresorbable, biosoluble or bioerodable refer to the property of a material or stent to degrade, absorb, resorb, or erode away from an implant site. Scaffolds may also be constructed of bioerodible metals and alloys. The scaffold, as opposed to a durable metal stent, is intended to remain in the body for only a limited period of time. In many treatment applications, the presence of a stent in a body may be necessary for a limited period of time until its intended function of, for example, maintaining vascular patency and/or drug delivery is accomplished. Moreover, it has been shown that biodegradable scaffolds allow for improved healing of the anatomical lumen as compared to metal stents, which may lead to a reduced incidence of late stage thrombosis. In these cases, there is a desire to treat a vessel using a polymer scaffold, in particular a bioabsorbable or bioresorbable polymer scaffold, as opposed to a metal stent, so that the prosthesis's presence in the vessel is temporary.
Polymeric materials considered for use as a polymeric scaffold, e.g. poly(L-lactide) (“PLLA”), poly(D,L-lactide-co-glycolide) (“PLGA”), poly(D-lactide-co-glycolide) or poly(L-lactide-co-D-lactide) (“PLLA-co-PDLA”) with less than 10% D-lactide, poly(L-lactide-co-caprolactone), poly(caprolactone), PLLD/PDLA stereo complex, and blends of the aforementioned polymers may be described, through comparison with a metallic material used to form a stent, in some of the following ways. Polymeric materials typically possess a lower strength to volume ratio compared to metals, which means more material is needed to provide an equivalent mechanical property. Therefore, struts must be made thicker and wider to have the required strength for a stent to support lumen walls at a desired radius. The scaffold made from such polymers also tends to be brittle or have limited fracture toughness. The anisotropic and rate-dependent inelastic properties (i.e., strength/stiffness of the material varies depending upon the rate at which the material is deformed, in addition to the temperature, degree of hydration, thermal history) inherent in the material, only compound this complexity in working with a polymer, particularly, bioresorbable polymers such as PLLA or PLGA.
An additional challenge with using a bioresorbable polymer (and polymers generally composed of carbon, hydrogen, oxygen, and nitrogen) for a scaffold structure is that the material is radiolucent with no radiopacity. Bioresorbable polymers tend to have x-ray absorption similar to body tissue. A known way to address the problem is to attach radiopaque markers to structural elements of the scaffold, such as a strut, bar arm or link. For example, FIG. 1 shows a link element 9d connecting a distal end ring 5d to an adjacent ring 5. The link element 9d has a pair of holes. Each of the holes holds a radiopaque marker 11. There are challenges to the use of the markers 11 with the scaffold 10. There needs to be a reliable way of attaching the markers 11 to the link element 9d so that the markers 11 will not separate from the scaffold during a processing step like crimping the scaffold to a balloon or when the scaffold is balloon-expanded from the crimped state. These two events—crimping and balloon expansion—are particularly problematic for marker adherence to the scaffold because both events induce significant plastic deformation in the scaffold body. If this deformation causes significant out of plane or irregular deformation of struts supporting, or near to markers the marker can dislodge (e.g., if the strut holding the marker is twisted or bent during crimping the marker can fall out of its hole). A scaffold with radiopaque markers and methods for attaching the marker to a scaffold body is discussed in US20070156230.
There is a need to improve upon the reliability of radiopaque marker securement to a scaffold for a thin-walled scaffold. Related to this need, there is a need to improve upon the performance characteristics of a scaffold, especially thin-walled scaffolds made from a bioresorbable material that must be navigated around tortuous anatomy.